This invention relates to volatile memory elements, and more particularly, to volatile memory elements for integrated circuits such as programmable integrated circuits.
Integrated circuits often contain volatile memory elements. Typical volatile memory elements are based on cross-coupled inverters (latches) and are used to store data. Each memory element can store a single bit of data.
Volatile memory elements are often used to store configuration data in programmable logic devices. Programmable logic devices are a type of integrated circuit that can be customized in relatively small batches to implement a desired logic design. In a typical scenario, a programmable logic device manufacturer designs and manufactures uncustomized programmable logic device integrated circuits in advance. Later, a logic designer uses a logic design system to design a custom logic circuit. The logic design system uses information on the hardware capabilities of the manufacturer's programmable logic devices to help the designer implement the logic circuit using the resources available on a given programmable logic device.
The logic design system creates configuration data based on the logic designer's custom design. When the configuration data is loaded into the memory elements of one of the programmable logic devices, it programs the logic of that programmable logic device so that the programmable logic device implements the designer's logic circuit. The use of programmable logic devices can significantly reduce the amount of effort required to implement a desired integrated circuit design.
Conventional memory cells contain clear transistors. The clear transistors can be turned on to set the contents of the memory cells to a known value. For example, the clear transistor in each cell may be used to selectively connect a storage node in the cell to ground, thereby clearing the cell contents. Memory cells can be cleared in this way during power-up operations or as part of a reconfiguration operation.
As circuit complexity increases in modern integrated circuits, it is becoming increasingly desirable to reduce the amount of area consumed by circuit components. For example, as the number of memory cells on certain integrated circuits is increased, it is desirable to ensure that cell sizes are minimized so that the total area of the integrated circuit does not become too large. At the same time, care should be taken to avoid incurring unusually large amounts of leakage current loses, as these losses can lead to unfavorably large amounts of power consumption.
It would therefore be desirable to be able to provide an integrated circuit such as a programmable logic device integrated circuit with memory cells that are compact and that exhibit satisfactory leakage current performance.